Polyimides are currently used to a large extent in microcircuit fabrication as carrier layers and insulating layers in electronic packaging of semiconductor chips and micro-circuitry. When the polyimides are provided on a substrate, such are normally applied as precursors in the polyamic acid form that contains free carboxylic acid groups. For the most part, the fabrication of photopatterned polyimide layers is carried out using non-photoreactive polyimide precursors with the aid of conventional photoresist materials.
In addition, in order to be able to significantly reduce the number of processing steps, there have been some suggestions to directly photopattern photocrosslinkable polyimide precursors. For instance, see U.S. Pat. Nos. 4,670,535 and 4,778,859.
Of particular importance in the electronics industry are polyimides based upon 3,3',4,4'-biphenyl tetracarboxylic acid dianhydride (BPDA) and p-phenylene diamine (PDA). Such polyimides possess several important properties for such purposes including low solvent uptake, low thermal expansion coefficient (TCE) and high glass transition temperature (Tg). However, developing a photosensitive polyimide system based on a BPDA and PDA backbone is made difficult because of high absorption of the backbone in much of the ultraviolet light region. This high absorption limits the efficiency of any photochemistry occurring at 313, 334, 365, 405 and 436 nanometers mercury lines because of competitive absorption between the backbone and any initiators required for the photochemistry.
Although polyimides have been obtained from 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 2,2-bis[4-aminophenyl] hexafluoropropane, these polymers do not possess the types of physical properties needed for use in the electronics industry such as a high glass transition temperature (&gt;350.degree. C.), a low thermal expansion coefficient and high solvent resistance. These polyimides and their related polyimide precursors; however, have much lower absorption in the UV region as compared to polyimides based on BPDA/PDA.